1. Field of the Invention
The present invention is directed to a method and to an apparatus for this method of displaying an image wherein the signals forming the image are obtained by irradiating an examination subject with a radiation field, such as ultrasound, and detecting the reflected (echo) signals.
2. Description of the Prior Art
A precautionary examination of the female mammary gland for early detection of breast cancer with imaging methods is extremely desirable since this illness, which represents the most common type of cancer in women in industrialized countries, has a significant tendency to spread and an early detection of the sickness usually means a cure.
By contrast to x-ray mammographic examinations, ultrasound examinations are completely non-hazardous and even extremely dense gland tissue (mastopathy) does not represent a problem since the tumors in dense gland tissue can be visibly identified in a sonographic display. X-ray mammography provides no diagnostic utility given patients with a mastopathy or with endoprotheses in the breast region since the tumors can then not be displayed or can only be poorly displayed.
By contrast thereto, however, many previous sonographic methods were incapable of exceeding x-ray mammography in the early detection of malignant tumors in the case of breast examination with respect to sensitivity and specificity. It is precisely this capability, however, that would be desirable since an ultrasound method would represent the ideal examination method because of the completely risk-free nature thereof.
An ultrasound examination is normally undertaken with an acoustic head (applicator) that the physician places onto the organ to be examined in order to thus acquire a tomogram which, as a so-called B-image, corresponds to a slice presentation. Methods are also known that superimpose a number of images registered from different directions on one another in the fashion of a computer tomograph.
A method and an apparatus for producing an image from ultrasound echoes is known from the article by K. Soetanto, "An in-vivo technique for estimation of size and relative sound velocity of breast tumor using distorted image in ultra-sonic tomogram", which appeared in Japanese Journal of Applied Physics, Vol. 24, No. 24-1, 1985, Tokyo (JP), pp.84-86. This describes a method for estimating the size and relative speed of sound in a breast tumor wherein a distorted ultrasound image is evaluated. The distortions arise because tumorous tissue has a different speed of sound than healthy tissue. The article describes investigations of a cylindrical member situated in a water tank, this member having a speed of sound therein different from the surrounding water and a reflector plate arranged therebehind. A linear distortion of the ultrasound image of the reflector plate arises because of, the different speeds of sound and the reflector plate in the ultrasound image has lateral tails because of, the refraction of the ultrasound waves is visible in the ultrasound image. The spacing of the tails from one another indicates the diameter of the cylinder. This perception is then used in ultrasound echo tomography of the female breast. The chest wall behind the tumor is used as the planar reflector. The ultrasound examination itself is implemented with a normal ultrasound apparatus, whereby the examining person can freely move the ultrasound transducer without auxiliaries.
In an ultrasound tomography apparatus disclosed by U.S. Pat. No. 4,509,368, transmitted and reflected signals are superimposed and correlated. Although this arrangement enables a gain in information compared to previously known solutions, systems operating according to this method have not been used in practice in significant numbers. It is thereby an impediment that the apparatus is relatively complicated in structure and that a plurality of acoustic transmitters and acoustic receivers are required, as a result of which the apparatus is expensive to acquire and is also not simple in terms of manipulation.
Further, German OS 40 37 387 discloses a method wherein the echo values obtained for coinciding spatial points from radiation directions opposite one another are superimposed, so that signal parts ultimately remain only for those spatial points that deviate from one another dependent on the radiation direction. As a result, information with respect to the shape and the surface structure of a recognized inhomogeneity can be derived better, since acoustic occlusions and the like are eliminated. It is still a disadvantage of this method, however, that the part of the body to be examined must be irradiated from two opposite spatial directions, so that the acoustic head must either be shifted correspondingly in location or two acoustic heads are required.
Published PCT Application 83/02053 also discloses an ultrasound scanner means for producing ultrasound tomograms of the female breast. The breast is thereby situated in an examination position that corresponds to the examination position in X-ray mammography, so that the ultrasound mammogram can be easily correlated with the X-ray image. The ultrasound scanner means includes an ultrasound-transmissive plate on which the breast is seated. A movable ultrasound transducer or an ultrasound array with which a complete scanning of the breast can be implemented is situated under the plate. Similar to X-ray mammography, an external compression with an air-filled or water-filled balloon or with sand bags or vacuum means as well can ensue. The means needed for compression, however, impede access in the upper breast region. It is noted with respect thereto in the reference that this region should remain free so that it can be brought into a shape wherein the visibility of details is optimized.